In many applications, the opening and closing of doors of heavy weight, e.g. hatch covers of ships, large-scale butterfly vanes, or other various mechanisms, require a moving angle of less than one rotation. Generally, a rotary actuator is employed for providing reciprocating rotation (oscillation) having a limited angle.
The conventional rotary actuator substantially comprises a casing having a stopper secured to an inner wall thereof, a stator and vanes, and the reciprocating rotation (oscillation) has been accomplished by alternately supplying oil under pressure into inlet and outlet ports. In actuators of this type, the pressurized oil has been known to leak from a clearance defined at the casing inner wall, and the desired driving force cannot be provided. To overome this problem, sealing material such as rubber, synthetic resin or the like has been attached to the vanes. However, the sealing material suffers from severe abrasion, or creates friction relative to an inner side of the casing thereby significantly reducing efficiency of the actuator in generating rotation torque. For these reasons the need has arisen for a rotary actuator which prevents oil leakage without using sealing material.